Process for operating a steam power plant with a coal-fired steam generator as well as a steam power plant

ABSTRACT

A process and apparatus for operating a steam power plant having a coal-fired steam generator, in which at least two partial air streams are heated by means of the exchange of heat in at least one exhaust gas/air heat exchanger with hot exhaust gases that are obtained in the steam generator by the combustion of coal. The coal is dried in a milling-drying system with a first partial air stream of the heated air supplied as a primary air stream and a second partial air stream of the heated air is supplied to the steam generator as a secondary air stream for the combustion of the coal. The secondary air stream is preheated in an air/air heat exchanger upstream from the exhaust gas/air heat exchanger, with respect to the direction in which the secondary air stream flows, by the exchange of heat with the heated primary air stream.

BACKGROUND

The disclosure relates to a process for operating a steam power plant with a coal-fired steam generator and to a steam power plant.

Processes for operating a steam power plant with a coal-fired steam generator as well as steam power plants having a coal-fired steam generator for performing said process are known. For example, the publication “Kraftwerkstechnik zur Nutzung fossiler, regenerativer und nuklearer Energiequellen” by Karl Strauβ, Springer-Verlag, 1994, refers to such a process and to such a steam power plant. The detailed operation of a coal-fired steam generator of such a steam power plant is described in section “4.3.2.2 Staubfeuerungen” [powder combustion systems] in the above publication. According to this publication the coal that is needed for the combustion system in the steam generator is ground into coal powder and dried in at least one coal mill. Preheated air that is generated in an air preheater through the exchange of heat from drawn-in fresh air with the hot exhaust gas or flue gas produced by the coal combustion system in the steam generator is used to dry the coal in the mill and to burn the coal powder in the combustion chamber of the steam generator.

In this process, the preheated partial stream of air that is transported to the coal mill as a drying and carrier medium is generally referred to as the primary air stream, and the preheated partial stream of air that is transported to the coal powder burners is generally referred to as the secondary air stream. Since only hot air or primary hot air that lies within specific temperature limits can be transported to the coal mill due to design and process engineering-related considerations, the feeding-in of cold air or fresh air is provided in order to control the temperature of the hot primary air stream downstream from the air preheater, in other words, downstream from the air preheater relative to the direction of flow of the primary air stream. Relative to the direction of flow of the fresh air, this supply of cold or fresh air is removed or branched off from the fresh air stream upstream from the air preheater, and, passing by the air preheater—in other words, in a bypass—it is transported to the hot primary air stream.

Since this primary cold air stream, which is required for temperature control, bypasses the air preheater, a worsening of the flue-gas or exhaust-gas-to-air ratio results, which leads to additional limitations with regard to the optimization of the air preheater and the cold end of the steam generator-in other words the exhaust gas outlet on the steam generator on the exhaust gas-flow side downstream from the air preheater—with respect to the exhaust gas temperature.

SUMMARY

The object of the disclosure is therefore to create a process for operating a steam power plant with a coal-fired steam generator that avoids the aforesaid disadvantages. In particular, an object of the disclosure is to create a process for operating a steam power plant with a coal-fired steam generator in which the flue-gas- or exhaust-gas-to-air ratio at the air preheater is improved, so that the lowest possible exhaust gas temperature is achieved at the cold end of the steam generator. A further object of the present disclosure is to propose a steam power plant for performing the process.

The solution in accordance with the disclosure creates a process for operating a steam power plant having a coal-fired steam generator as well as a steam power plant having a coal-fired steam generator to perform such a process, which has the following advantages:

-   -   i. Increasing the steam generator efficiency accomplished by         lowering the flue gas or exhaust gas temperature while         maintaining feasible temperature differences at the air         preheater and therefore utilizing the steam generator more         efficiently.     -   ii. Reducing the amount of CO2 discharged into the atmosphere by         increasing the efficiency of the steam generator.

In one embodiment of the disclosure at least part of the heated primary air stream is passed by the air/air heat exchanger by means of a bypass line that circumvents the air/air heat exchanger, and the bypassed partial amount of the heated primary air stream is controlled by means of a control device. With this measure it is possible to affect the temperature control of the hot primary air stream and to meet the requirements for the necessary temperatures in the milling-drying system.

In one embodiment of the disclosure a partial primary air stream that, with respect to the direction of flow of the primary air, is removed upstream from the exhaust gas/air heat exchanger is mixed with the heated primary air stream downstream from the air/air heat exchanger relative to the direction of flow of the primary air stream, and this partial stream of primary air is controlled with the aid of a control device. By adding this cold partial stream of primary air to the heated primary air stream, a temperature range can be set for the heated primary air stream, so that the coal mill or milling-drying system can be operated in accordance with its design from a design and process engineering standpoint.

In one embodiment of the disclosure the secondary air stream is preheated further, with respect to the direction of its flow downstream from the air/air heat exchanger and upstream from the exhaust gas/air heat exchanger, and/or the primary air stream with respect to the direction of its flow upstream from the exhaust gas/air heat exchanger and downstream from the point at which the exhaust gas/air heat exchanger bypass line branches off, is preheated further by means of the exchange of heat with the steam or hot water removed from the steam circuit of the steam power plant or from a third-party steam source. By means of this measure, for example, the heating panel temperature of the exhaust gas/air heat exchanger at the cold end of the exhaust gas/air heat exchanger can be increased to a particular temperature, so that the heating panel of the exhaust gas/air heat exchanger is not placed at risk due to the acid dew point of the flue and exhaust gas.

In a further embodiment of the disclosure, in the arrangement of two exhaust gas/air and heat exchangers, the two heat exchangers are connected in parallel to each other, and in the first heat exchanger the primary air stream is heated by means of an initial controlled partial stream of exhaust gas, and in the second heat exchanger the secondary air stream is heated by means of a second controlled partial stream of exhaust gas. With this arrangement, the primary and secondary air streams may each be heated separately in separate exhaust gas/air heat exchangers. This allows the two exhaust gas/air heat exchangers to be designed in a simple manner to meet the requirements with respect to necessary transfer of heat to the two air streams. In a further embodiment of the disclosure, the two exhaust gas/air heat exchangers have different transfer capacities, sizes, and design types.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure may be better understood and its numerous objects and advantages will become apparent to those skilled in the art by reference to the accompanying drawings in which:

FIG. 1 is a schematic diagram of the fresh air, flue gas, and exhaust gas path at the cold end of the steam generator of a first embodiment of a steam power plant; and

FIG. 2 is a schematic diagram of the fresh air, flue gas, and exhaust gas path at the cold end of the steam generator of a second embodiment of a steam power plant.

DETAILED DESCRIPTION

FIG. 1 shows a schematic diagram of the coal-fired steam generator 2 of a steam power plant 1, which is not otherwise shown. In particular, the cold end of the steam generator 2 is shown with respect to the flue gas or exhaust gas path. The cold end of the steam generator 2 refers to the position or point inside of the steam generator 2 at which the exhaust gas stream 21 leaves the steam generator 2 before it generally is cleaned in various cleaning devices that are not shown and then discharged into the atmosphere. As the expression indicates, at the cold end of the steam generator 2 the exhaust gas stream 21 has had most of its heat drawn off, as explained in greater detail below.

The exhaust gas 21 that is produced through the combustion of coal 26 that has been reduced in size in a milling/drying system 3 in the combustion chamber 20 of the steam generator 2 transfers a large part of its heat content to a working medium within the steam generator 2. This medium circulates in the steam power plant 1 and constitutes the water/steam circuit. Its energy content is used to generate electrical power by the steam turbine and generator. The exhaust gas stream 21, which is partially cooled, is then transported through an exhaust gas/air heat exchanger 7, in which an additional portion of the heat content of the exhaust gas stream 21 is transferred to a primary air stream 23 and a secondary air stream 24, the primary air stream flow path 28 within the heat exchanger 7 being separate from the secondary air stream flow path 29. The exhaust gas stream 21 is drawn by a suction blower 4 from the combustion chamber 20 of the steam generator 2 through the exhaust gas line 10.

The air or fresh air 22 that is needed to burn coal 26 is supplied by means of a fresh air blower 5 and a fresh air line 11. With respect to the direction in which the fresh air stream 22 flows, this stream 22 is divided downstream from the fresh air blower 5 into a primary air stream 23 and a secondary air stream 24, and the pressure of the primary air stream 23 is increased for reasons of processing technology (in particular: the milling-drying system 3) relative to the pressure of the secondary air stream 24 by means of a separate primary air blower 6. The primary air stream 23 as well as the secondary air stream 24 are each independently directed through the exhaust gas/air heat exchanger 7, in which they are heated by means of the exchange of heat with the hot exhaust gas stream 21. The exhaust gas/air heat exchanger 7 may be an air preheater, which is known per se, in particular a rotating regenerative air preheater.

The secondary air stream 24 is fed via a secondary air line 13 to the exhaust gas/air heat exchanger 7 and to the burners 19, from which it travels to the combustion chamber 20 together with the coal 26 in order to combust the coal. The primary air stream 23 is fed via a primary air line 12 to the exhaust gas/air heat exchanger 7 and to a milling-drying system 3. In the milling-drying system 3 the heated primary air 23 is used to dry the coal and as a carrier medium for the coal powder 26 that is milled in the milling system 3. The coal powder 26 is fed by means of the hot primary air stream 23 through the coal powder lines 18 to the burners 19, and the hot primary air stream 23 as well as the hot secondary air stream 24 contribute to the combustion of the coal 26.

In the disclosure, part of the heat from the primary air stream 23 that is heated in the exhaust gas/air heat exchanger 7 is transferred to the cold secondary air stream 24. This is accomplished by means of an air/air heat exchanger 8 through which the primary air stream 23 and the secondary air stream 24 flow, and which, with respect to the direction of the secondary air flow 24 is located upstream from the exhaust gas/air heat exchanger 7. As a result of this measure, the cold secondary air stream 24 is subjected to preheating by the hot primary air stream 23 before the secondary air stream 24 is heated further in the exhaust gas/air heat exchanger 7. The use of part of the heat of the primary air stream 23 that is heated in the exhaust gas/air heat exchanger 7 to preheat the cold secondary air stream 24 also results in further removal of heat from the hot exhaust gas stream 21 in the exhaust gas/air heat exchanger 7. The further removal of heat from the exhaust gas stream ultimately means that there is a further lowering of the temperature of the hot exhaust gas stream 21 and, therefore, an increase in the efficiency of the steam generator.

Since in the solution of the disclosure the entire primary cold air 23 passes through the exhaust gas/air heat exchanger 7, the total amount of air (primary and secondary air 23, 24) that passes through the exhaust gas/air heat exchanger 7 is increased, improving the exhaust gas/air ratio in comparison with the prior-art arrangement in which a portion of the primary cold air stream 23 branches off, with respect to the direction in which the primary cold air stream flows, upstream from the exhaust gas/air heat exchanger and, with respect to the direction in which the heated primary air stream 23 flows, is added to upstream from the milling-drying system 3 as a temperature-conditioning medium to the heated primary air stream 23. In other words, the exhaust gas/air ratio is the quantitative ratio of the exhaust gas stream that gives off heat to the air stream that absorbs this heat. Therefore, for a person skilled in the art the exhaust gas/air ratio is an indicator or measure of the quantitative yield of the exhaust gas heat content.

As a further positive side effect of the increase in efficiency, the heat transferred by the hot primary air stream 23 to the cold secondary air stream 24, the temperature of the hot primary air stream 23 is conditioned by the exchange of heat, i.e. cooled, and therefore a temperature-conditioned, i.e. cooled, primary air stream 23 is supplied to the milling-drying system 3. This is important, because the milling-drying system 3 may and should only be operated at certain primary air temperatures for mechanical reasons and for reasons relating to the technical process. If the required temperature of the primary air stream 23 is not reached as a result of the exchange of heat in the air/air heat exchanger 8, cold primary air 23 can be mixed with the primary air stream 23 that is still too hot upstream from the milling-drying system 3 with respect to flow by means of a bypass line 14. This cold primary air 23 is once again taken from the primary air line 12 upstream from the exhaust gas/air heat exchanger 7 with respect to flow. The amount of the cold primary air partial stream 23.1 that is mixed in may be controlled by a control device 16, typically a butterfly-type control valve. Another option for controlling the temperature of the hot primary air stream 23 on the way to the milling-drying system 3 is to route a partial flow of the hot primary air stream 23 past the air/air heat exchanger 8 by means of a bypass line 15, in other words to bypass the unit, so that only a certain portion of the heat from the hot primary air stream 23 is transferred to the cold secondary air stream 24. The amount of the hot primary air stream 24 that is routed through the bypass line 15 once again may be controlled by a control device 17, typically a butterfly-type control valve.

In order to preheat the cold secondary air stream 24 further, with respect to the direction of flow of the secondary air stream 24, upstream from the exhaust gas/air heat exchanger 7 and downstream from the air/air heat exchanger 8, a steam or hot water/air preheater 9 may be disposed. In this preheater or heat exchanger 9, the secondary air stream 24 is heated either by steam or by hot water, and the steam or the hot water is obtained either from the water/steam circuit in the steam power plant 1 or from an outside source.

It is also possible to preheat the cold primary air stream 23. To accomplish this, a steam or hot water/air preheater 9.1 is disposed upstream from the exhaust gas/air heat exchanger 7 with respect to the direction of flow of the primary air stream 23. In this preheater or heat exchanger 9.1, the primary air stream 23 is preheated, either by steam or by hot water, and the steam or the hot water is obtained either from the water/steam circuit in the steam power plant 1 or from an outside source.

FIG. 2 shows a second embodiment of a steam power plant 1 of the disclosure. The schematic path of the fresh, primary, and secondary air stream, 22, 23, 24, or the schematic arrangement of their lines, 11, 12, 13, as well as of the bypass lines, 14, 15, remains unchanged relative to the schematic path of the corresponding air streams and their lines shown in FIG. 1. The difference is that the primary air stream 23 and the secondary air stream 24 are no longer heated in an exhaust gas/air heat exchanger 7 by means of the exchange of heat with the hot exhaust gas stream 21, but rather are each heated in isolation, i.e. separately, in two exhaust gas/air heat exchangers 7, 25. For this purpose, the exhaust gas stream 21 is divided into two exhaust gas partial streams 21.1, 21.2 upstream from the exhaust gas/air heat exchanger 7, 25 with respect to flow, and each exhaust gas partial stream 21.1, 21.2 is passed through an exhaust gas/air heat exchanger 7, 25. In order to control the amount of the two exhaust gas partial streams 21.1, 21.2 in their exhaust gas lines 10.1, 10.2, a control device 27.1, 27.2, normally a butterfly-type control valve, may be used within the exhaust gas line 10.1, 10.2. The control device 27.1, 27.2 is advantageously disposed downstream from the exhaust gas/air heat exchanger 7, 25 with respect to the direction of flow of the exhaust gas partial flow 21.1, 21.2.

The two exhaust gas/air heat exchangers 7, 25 may be of the same design type, for example rotating regenerative air preheaters, or they may be of different design types. For example: one may be a rotating regenerative air preheater, and the other may be a tubular air preheater.

Depending on the heating requirements of the respective primary air stream 23 or secondary air stream 24 in the exhaust gas/air heat exchanger 7, 25, the two heat exchangers 7, 25 may be based on different concepts or designs with respect to their size or their heat transfer capacity. 

1. A process for operating a steam power plant having a coal-fired steam generator, the process comprising: heating at least first and second partial air streams in at least one exhaust gas/air heat exchanger by the transfer of heat from hot exhaust gases produced in a steam generator by the combustion of the coal, producing a heated primary air stream and a heated secondary air stream; drying the coal with the heated primary air stream in a milling-drying system; supplying the heated secondary air stream to the steam generator for the combustion of the coal; and preheating the second partial air stream in an air/air heat exchanger by a transfer of heat from the heated primary air stream, the air/air heat exchanger being upstream to the exhaust gas/air heat exchanger relative to a direction of flow of the second partial air stream.
 2. The process of claim 1 further comprising: transporting a bypassed partial amount of the heated primary air stream around the air/air heat exchanger with a bypass line that bypasses the air/air heat exchanger; and controlling the bypassed partial amount of the heated primary air stream with a control device.
 3. The process of claim 1 further comprising: removing a portion of the first partial air stream at a position that is upstream from the exhaust gas/air heat exchanger with respect to a direction of flow of the first partial air stream; adding the portion of the first partial air stream with the heated primary air stream at a position downstream from the air/air heat exchanger with respect to the direction of flow of the heated primary air stream; and controlling the portion of the first partial air stream with a control device.
 4. The process of claim 1 further comprising: providing steam or hot water from a water/steam circuit of the steam power plant or from an source external to the steam power plant to a secondary preheater located intermediate the air/air heat exchanger and the exhaust gas/air heat exchanger in the direction of flow of the second partial air stream; and preheating the second partial air stream by the transfer of heat from the steam or the hot water in the secondary preheater.
 5. The process of claim 3 further comprising: providing steam or hot water from a water/steam circuit of the steam power plant or from an source external to the steam power plant to a primary preheater located intermediate the exhaust gas/air heat exchanger and the position where the portion of the first partial air stream is removed, in the direction of flow of the first partial air stream; and preheating the first partial air stream by the transfer of heat from the steam or the hot water in the primary preheater.
 6. The process of claim 1 wherein heating at least first and second partial air streams comprises transferring heat from a first controlled exhaust gas partial flow to a first partial air stream in a first exhaust gas/air heat exchanger, producing a heated primary air stream; and transferring heat from a second controlled exhaust gas partial flow to a second partial air stream in a second exhaust gas/air heat exchanger, producing a heated secondary air stream.
 7. The steam power plant comprising: a coal-fired steam generator; at least one exhaust gas/air heat exchanger having a gas side and an air side, the gas side receiving hot exhaust gas from the steam generator, the air side having first and second air stream flow paths, the first air stream flow path being separate from the second air stream flow path, a first air stream being received at an inlet of the first air stream flow path, a primary air stream being discharged at an outlet of the first air stream flow path, a second air stream being received at an inlet of the second air stream flow path, a secondary air stream being discharged at an outlet of the second air stream flow path; an air/air heat exchanger having a first inlet in fluid communication with the exhaust gas/air heat exchanger first air stream flow path outlet receiving the primary air stream and a first outlet discharging the primary air stream, a second inlet receiving the first air stream and a second outlet in fluid communication with the exhaust gas/air heat exchanger second air stream flow path inlet; and at least one milling-drying system having an inlet in fluid communication with the air/air heat exchanger first outlet and an outlet in fluid communication with the steam generator.
 8. The steam generating system of claim 7 further comprising: a primary air line including: a first segment providing fluid communication between the exhaust gas/air heat exchanger first air stream flow path outlet and the air/air heat exchanger first inlet, a second segment providing fluid communication between the air/air heat exchanger first outlet and the at least one milling-drying system inlet, and a first bypass line providing fluid communication between the first and second segments; and a first control device controlling flow through the first bypass line.
 9. The steam generating system of claim 8 wherein the primary air line also includes a third segment providing fluid communication with the exhaust gas/air heat exchanger first air stream flow path inlet and a second bypass providing fluid communication between the third segment and the second segment, the steam generator system further comprising a second control device controlling flow through the second bypass line.
 10. The steam generating system of claim 7 further comprising a steam or hot water/air heat exchanger in fluid communication with the air/air heat exchanger second outlet and the exhaust gas/air heat exchanger second air stream flow path inlet.
 11. The steam generating system of claim 9 further comprising a steam or hot water/air heat exchanger disposed in the third segment of the primary air line intermediate the exhaust gas/air heat exchanger and the second bypass.
 12. The steam generating system of claim 7 wherein the at least one exhaust gas/air heat exchanger includes first and second exhaust gas/air heat exchangers, each of the exhaust gas/air heat exchangers having a gas side and an air side, the air side of the first exhaust gas/air heat exchanger including the first air steam flow path and the air side of the second exhaust gas/air heat exchanger including the second air stream flow path.
 13. The steam generating system of claim 12 wherein each of the exhaust gas/air heat exchangers has a heat transfer capacity, the heat transfer capacity of the first exhaust gas/air heat exchanger being different from the heat transfer capacity of the second exhaust gas/air heat exchanger.
 14. The steam generating system of claim 12 wherein each of the exhaust gas/air heat exchangers has a design type, the design type of the first exhaust gas/air heat exchanger being different from the design type of the second exhaust gas/air heat exchanger.
 15. The steam generating system of claim 12 wherein each of the exhaust gas/air heat exchangers has a design size, the design size of the first exhaust gas/air heat exchanger being different from the design size of the second exhaust gas/air heat exchanger. 